Constraints on the Geometry of Circumstellar Envelopes: Optical Interferometric and Spectropolarimetric Observations of Seven Be Stars

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Abstract
We have obtained contemporaneous optical interferometry and spectropolarimetry of seven Be stars. The interferometry was done using both continuum and narrow Hα line filters to observe the circumstellar envelope emission. The envelopes of all seven stars were resolved interferometrically in Hα. Of these seven stars, the envelopes of four were not circularly symmetric and showed clear evidence of elongation. The position angles of the major axis of the elongation were in good agreement with the disk orientation inferred from the intrinsic polarization data, which samples material within a few stellar radii, and also agreed with previous results from the radio, which samples material out as far as 100 stellar radii but at lower resolution. This finding indicates that the envelope alignment persists over 2 orders of magnitude in radius. Minimum inclination angle estimates from the interferometry are presented for six of the seven stars. Under the assumption that the envelopes are fairly thin circularly symmetric disks, there is a straightforward explanation of the interferometric results. The stars with the greatest elongations, ζ Tau, Per, and ψ Per, are seen nearly equator-on; γ Cas is seen at a more intermediate latitude, consistent with previous results; and 48 Per and η Tau, which are only slightly asymmetric, are nearly pole-on. β CMi had insufficient coverage to determine whether its envelope is asymmetric. These results are consistent with independent indications of the inclinations based on the polarimetry. Spectral diagnostics have been used in the past to classify Be stars as pole-on or as shell stars (usually interpreted to indicate an equator-on orientation). Our inclination angle for 48 Per is consistent with its pole-on classification by Slettebak, and the three equator-on stars have all been previously classified as shell stars. However, both γ Cas and η Tau have also been previously classified as shell stars, but we find that they have intermediate and pole-on orientations, respectively. We interpret this finding as evidence that, while equator-on stars may preferentially be shell stars, not all shell stars are actually equator-on. Our results show that the Hα emission region extends up to about 12 stellar radii, possibly depending on spectral type. The size of this region correlates with the equivalent width of the Hα emission. Our results also provide strong constraints for allowed models of Be star envelopes. In particular, for ζ Tau, we derive an upper limit to the disk opening angle of 20°, which limits the vertical extent of the envelope. We also find that the polarization position angle is perpendicular to the interferometric major axis in all cases. This rules out envelope models that are both optically and geometrically thick, since these models produce polarization parallel to the plane of the disk. We conclude that results from the combined interferometry and polarimetry strongly favor the disk paradigm for Be stars over mildly ellipsoidal models.